Newt Gingrich and Mitt Romney, who have spent much of this year ignoring each other, are now on a collision course in the contest for the Republican presidential nomination, facing a series of battles in January that will possibly decide the race by testing which one can best surmount his own weaknesses.

Whatever assumptions may have prevailed a month or two ago have been significantly altered by increasing support for Gingrich.

"There is no longer a scenario where the nomination will default to Romney," said Steve Schmidt, a Republican strategist who was a top adviser to the campaign of John McCain in 2008.

Gingrich's dramatic rise - he now leads national and some state polls, including a new Washington Post-ABC News poll in Iowa - has put serious and unexpected pressure on Romney to adapt and intensify his campaign, which to this point has been both disciplined and unexciting. Steady-as-you-go remains the operative phrase from his Boston headquarters, but other Republican strategists say tactical changes are afoot to deal with the Gingrich threat.

Those changes include a sharper economic message, implicit and explicit contrasts with Gingrich and greater openness with the media. Romney, who has struggled to rally rank-and-file Republicans, is also likely to play on establishment GOP fears about Gingrich as the party's nominee against President Obama.

Gingrich, in turn, is scrambling to build, almost from scratch, a campaign operation that can match the good fortune of his newfound support. Lacking the funds or the infrastructure of the Romney campaign, Gingrich is looking for a quick infusion of money while urgently adding staff to handle the demands of building organizations in many states at once.

One example of the challenges he faces could be seen early Tuesday evening at his Iowa campaign headquarters in suburban Des Moines. Dozens of cell phones, newly arrived, were being charged in the mostly empty space. Just two volunteers - a woman and her mother, both from out of state - were making calls. A campaign official said more help is on the way.

The Republican nomination contest could last for months, and some senior Republican leaders say it is still too early to call the competition a two-person race. The answers to both questions will come in January and early February, when five states are scheduled to hold contests: Iowa, New Hampshire, South Carolina, Florida and Nevada. Romney is favored in New Hampshire and Nevada; Gingrich is now the favorite in Iowa and South Carolina.

Of the two campaigns, Romney's has been preparing far longer for a potentially lengthy nomination battle. But that means Romney cannot allow Gingrich to embarrass him in January.

As one strategist put it, "Romney cannot afford to lose two in a row. Thus, Florida is key."